3.16.54-rc1 review patch. If anyone has any objections, please let me know.
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From: Hou Tao houtao1@huawei.com
commit b9a41d21dceadf8104812626ef85dc56ee8a60ed upstream.
The following BUG_ON was hit when testing repeat creation and removal of DM devices:
kernel BUG at drivers/md/dm.c:2919! CPU: 7 PID: 750 Comm: systemd-udevd Not tainted 4.1.44 Call Trace: [<ffffffff81649e8b>] dm_get_from_kobject+0x34/0x3a [<ffffffff81650ef1>] dm_attr_show+0x2b/0x5e [<ffffffff817b46d1>] ? mutex_lock+0x26/0x44 [<ffffffff811df7f5>] sysfs_kf_seq_show+0x83/0xcf [<ffffffff811de257>] kernfs_seq_show+0x23/0x25 [<ffffffff81199118>] seq_read+0x16f/0x325 [<ffffffff811de994>] kernfs_fop_read+0x3a/0x13f [<ffffffff8117b625>] __vfs_read+0x26/0x9d [<ffffffff8130eb59>] ? security_file_permission+0x3c/0x44 [<ffffffff8117bdb8>] ? rw_verify_area+0x83/0xd9 [<ffffffff8117be9d>] vfs_read+0x8f/0xcf [<ffffffff81193e34>] ? __fdget_pos+0x12/0x41 [<ffffffff8117c686>] SyS_read+0x4b/0x76 [<ffffffff817b606e>] system_call_fastpath+0x12/0x71
The bug can be easily triggered, if an extra delay (e.g. 10ms) is added between the test of DMF_FREEING & DMF_DELETING and dm_get() in dm_get_from_kobject().
To fix it, we need to ensure the test of DMF_FREEING & DMF_DELETING and dm_get() are done in an atomic way, so _minor_lock is used.
The other callers of dm_get() have also been checked to be OK: some callers invoke dm_get() under _minor_lock, some callers invoke it under _hash_lock, and dm_start_request() invoke it after increasing md->open_count.
Signed-off-by: Hou Tao houtao1@huawei.com Signed-off-by: Mike Snitzer snitzer@redhat.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/md/dm.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-)
--- a/drivers/md/dm.c +++ b/drivers/md/dm.c @@ -2912,11 +2912,15 @@ struct mapped_device *dm_get_from_kobjec
md = container_of(kobj, struct mapped_device, kobj_holder.kobj);
- if (test_bit(DMF_FREEING, &md->flags) || - dm_deleting_md(md)) - return NULL; - + spin_lock(&_minor_lock); + if (test_bit(DMF_FREEING, &md->flags) || dm_deleting_md(md)) { + md = NULL; + goto out; + } dm_get(md); +out: + spin_unlock(&_minor_lock); + return md; }